Why would Trump challenge Clinton to take a drug test?

In a stunning new twist to the brutal White House race, one presidential nominee is challenging the other to take a drug test.

While under fire from numerous sexual assault allegations, Republican presidential nominee Donald Trump on Saturday suggested his rival Democratic presidential nominee Hillary Clinton was "pumped up" on performance-enhancing drugs during their last presidential debate.

The unsubstantiated attack from the Republican nominee came as he accused "corrupt" media of seeking to rig November's vote in Clinton's favour, by reporting on the recent claims of his sexual misconduct.

Trump has trampled all conventions in his treatment of his opponent, vowing that if elected he would jail the Democratic nominee over her email practices as secretary of state — and making "Lock Her Up" a rallying cry for his supporters.

His campaign has actively fueled right-wing conspiracy theories about Clinton's health, seizing on her bout of pneumonia last month to suggest she is concealing a major health problem, and is unfit for office.

He called for Clinton to be tested ahead of their final duel Wednesday in Las Vegas.

"I don't know what is going on with her," the 70-year-old told a rally in Portsmouth, New Hampshire.

"At the beginning of her last debate, she was all pumped up at the beginning. At the end, it was like, 'Take me down,' she could barely reach her car."

"Athletes, they make them take a drug test. I think we should take a drug test prior to the debate. Why don't we do that?" Trump said.

Some had another theory for Trump's accusations.

Saturday's broadside against his Democratic rival marked yet another escalation of Trump's scorched-earth electoral strategy heading into the final weeks of a race that has defied all political norms.

Steal the election

As the Manhattan billionaire tanks in the polls — abandoned by part of his own camp — he has spent the week claiming the media and a "global elite" are working against him, alleging that Clinton plotted to destroy the sovereignty of the United States.

"Hillary is running for president in what looks like a rigged election," he charged in New Hampshire.

Since the now-infamous 2005 Hollywood Insider tape showing Trump making lewd comments about women surfaced last week, ten women have come forward to say they were the victim of unwanted advances by the real estate mogul.

After issuing a statement condemning her husband's 2005 comments, Melania Trump has been notably absent from the campaign, and has yet to weigh in on the sexual assault allegations.

Trump's latest accuser, 63-year-old Cathy Heller, told The Guardian that he had grabbed and kissed her against her wishes during their first and only meeting 20 years ago.

Trump staunchly denies the women's allegations, insisting in one of a barrage of tweets to his 12 million followers:

The Republican also issued a statement on his campaign's website, denying another allegeation brought on from Summer Zervos, a former contestant on his reality TV show The Apprentice.

A confident Clinton has meanwhile scaled back her campaign commitments, keeping a low profile as her rival battles the incendiary allegations, triggered by the release, last week, of a video of him bragging about groping women.

But the Clinton camp issued a swift response to Trump's latest comments on the election, accusing him of seeking to erode public faith in the vote.

"This election will have record turnout, because voters see through Donald Trump's shameful attempts to undermine an election weeks before it happens," her campaign manager Robby Mook said in a statement.

The nation's top elected Republican, House speaker Paul Ryan, who last week declared that he would no longer "defend" the party's shoot-from-the-hip nominee, also rebuked Trump over his comments questioning the validity of the election process.

"Our democracy relies on confidence in election results, and the speaker is fully confident the states will carry out this election with integrity," said a statement issued late Saturday by Ryan's spokeswoman AshLee Strong.

The virulence of Trump's attacks on the Clinton camp has raised concerns about whether the real estate mogul would even acknowledge a defeat, and how his legions of supporters would react should he lose.

President Barack Obama -- who along with First Lady Michelle Obama stepped up this week as a heavy-hitting surrogate for Clinton -- echoed those concerns at a rally Friday, warning democracy itself was at stake in next month's vote.

"This is somebody who ... is now suggesting that if the election doesn't go his way, it's not because of all the stuff he's said, but it's because it's rigged and it's a fraud," said the US leader, whose second term ends on January 20.

"In a democracy, you have a contest, but if you lose then you say congratulations and you move on."